A lithographic apparatus applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A lithographic apparatus generally includes an illumination system. The illumination system receives radiation from a source, for example a laser, and produces a radiation beam suitable for illuminating a patterning device. Within a typical illumination system, the radiation beam is shaped and controlled such that at a pupil plane the radiation beam has a desired spatial intensity distribution, also referred to as an illumination mode. Examples of types of illumination modes are conventional, dipole, asymmetric, quadrupole, hexapole and annular illumination modes. The spatial intensity distribution at the pupil plane effectively acts as a secondary radiation source to produce the radiation beam. Following the pupil plane, the radiation is typically focused by an optical element (e.g., lens) group referred to hereafter as “coupling optics”. The coupling optics couples the focused radiation into an integrator, such as a quartz rod. The function of the integrator is to improve the homogeneity of the spatial and/or angular intensity distribution of the radiation beam. The spatial intensity distribution at the pupil plane is converted to an angular intensity distribution at the integrator, because the pupil plane substantially coincides with the front focal plane of the coupling optics.
Controlling the spatial intensity distribution at the pupil plane can be done to improve processing latitudes when an image of a patterning device is projected onto a substrate. In particular, spatial intensity distributions with dipolar, annular or quadrupole off-axis illumination modes have been proposed to enhance the resolution and/or other parameters of the projection, such as sensitivity to projection system aberrations, exposure latitude and depth of focus.